1. Field of the Invention
The present invention relates to a method and an apparatus for forming an image with multiple beams.
2. Description of the Related Art
An image forming apparatus that employs an electrophotographic method forms images in the following manner. A static electric charge formed on a photosensitive drum is exposed by using a semiconductor laser thereby forming an electrostatic latent image on the photosensitive drum, and the electrostatic latent image is then developed with a developer. In a conventional semiconductor laser, one semiconductor element emits one to four laser beams or at most eight laser beams. Recently, surface emitting lasers referred to as a vertical cavity surface emitting lasers (VCSEL) have been made commercially available, and put to practical use. Moreover, in recent years there have been developed image forming apparatuses that form images at high resolution and high speed by using the VCSELs.
For example, Japanese Patent Application Laid-open No. 2007-249172 (Patent document 1) discloses an image forming apparatus (an optical writing system) that uses the VCSEL. In this image forming apparatus, as shown in FIG. 40, a light source unit 1001 is composed of a semiconductor laser array in which a plurality of light sources (a plurality of semiconductor lasers) are arranged in a lattice-like pattern, or a surface emitting laser in which a plurality of light sources (a plurality of VCSELs or a plurality of surface-emitting semiconductor lasers) are arranged on the same chip in a lattice-like pattern. The arrangement position and orientation of the light source unit 1001 is adjusted so that the light source unit 1001 is tilted at an angle θ to a rotating shaft of a deflector such as a polygon mirror.
In FIG. 40, rows of the array of the light sources are denoted by “a”, “b”, and “c” from the top, and columns are denoted by “1”, “2”, “3”, and “4” from the left. Each of the light sources is denoted by a combination of a row number and a column number of the array. For example, the top-left light source is denoted by “a1”. The light source unit 1001 is arranged at the angle θ, so that, for example, the light sources a1 and a2 respectively expose a different position to be scanned from each another. Now assume that, as shown in FIG. 40, any two light sources from among those light sources are used to form an image equivalent to one pixel (1 pixel). For example, the light sources a1 and a2 compose an image equivalent to 1 pixel, and the light sources a3 and a4 compose another image equivalent to 1 pixel. In this manner, six pixels illustrated on the extreme right in FIG. 40 are composed by the light sources a1 to c4. Furthermore, it is assumed that a vertical direction in the plane of the drawing corresponds to a sub-scanning direction, and a center-to-center distance between the adjacent pixels respectively composed by each two of the light sources is equivalent to 600 dots per inch (dpi). In other words, a center-to-center distance between adjacent light sources in a row is equivalent to 1200 dpi. Namely, the light-source density is twice as much as the pixel density. Therefore, by changing a light-intensity ratio of the light sources, a position of a gravity center of the pixel can be shifted in the sub-scanning direction. Consequently, it is possible to form an image with high accuracy.
On the other hand, a VCSEL can emit about forty laser beams from one chip. Therefore, it is possible to form images at high resolution and high speed by employing the VCSEL in image forming apparatuses to form latent images. This is obvious from a technology disclosed in Patent document 1. When employing the VCSEL as a laser device for forming latent images, simple replacement of a semiconductor laser with the VCSEL does not lead to formation of sufficiently high-resolution latent images. For example, the VCSEL generates multiple laser beams in a planar form from a predetermined light-emitting region. In a laser device used for forming latent images, it is necessary to control a light intensity of a laser beam emitted from the laser device to a target light intensity. Particularly, in the case of the VCSEL, the degree of integration of laser beams is high in the light-emitting region, so that, to form a high-resolution latent image stably, it is necessary to control a light intensity of each of the laser beams.
Thus, the number of laser beams to be controlled is larger in the case of the VCSEL than in the case of the semiconductor laser. Therefore, it obviously takes a longer time to control the light intensities of the laser beams in the case of the VCSEL than in the case of the semiconductor laser. Consequently, high-speed image formation can not necessarily be achieved with the VCSEL. If the light-intensity control of each of the laser beams is skipped so as to achieve the high-speed image formation, it becomes difficult to achieve high-resolution image formation.
Various technologies have been developed to solve this problem. For example, Japanese Patent Application Laid-open No. 2007-021826 (Patent document 2) discloses an image forming apparatus and an optical writing device including a plurality of light-emitting elements and a light-emitting light source unit. The light-emitting light source unit includes a light-intensity detecting element that detects an intensity of a light emitted from each of the light-emitting elements. To control a light intensity of each of optical beams, the optical writing device disclosed in Patent document 2 further includes a number of volume resistances corresponding to the number of the beams and a sample-and-hold capacitor. By using the method disclosed in Patent document 2, it is possible to control a light intensity of each of multiple laser beams. However, a circuit size of a control circuit itself of the VCSEL increases. In addition, each of the volume resistances needs to be set to adjust the light intensity the number of times corresponding to the number of the laser beams to be emitted, so that the work efficiency lowers, and thus the frequency of maintenance increases.
Furthermore, Japanese Patent Application Laid-open No. 2005-161790 (Patent document 3) discloses a control method for light-intensity control. In this method, a first measuring unit separates each of optical beams output from a light source into a first optical beam and a second optical beam, and measures a light intensity of the first optical beam. A light-intensity control unit controls a light intensity of each of the optical beams so that a measurement result by the first measuring unit, i.e., the light intensity of the first optical beam becomes a light intensity indicated in a light-intensity command signal. A light intensity of the second optical beam is measured. A light-intensity correction value of each of the optical beams for substantially equalizing the light intensity of the second optical beam in a plurality of the optical beams is obtained based on a measurement result of the light intensity of the second optical beam. The obtained light-intensity correction value is stored.
The method disclosed in Patent document 3 can be used to control light intensities in a VCSEL. Because an image is formed while correcting the light intensity of each of the laser beams, it is possible to perform feedback at sufficiently high speed only if the number of the laser beams is not many. However, as in the case of the VCSEL, in which a great number of laser beams are emitted, there can be situations where it is not possible to perform feedback at a sufficiently-high efficiency with respect to the control of a light intensity of each of the laser beams within a scanning time during the image formation with consideration for an environmental variation of the VCSEL. Furthermore, when semiconductor laser elements composing the VCSEL cannot provide a predetermined light intensity with an initially-set correction range, it is not possible to complete the image formation with preventing a currently-formed image from being critically affected, and not possible to correct the light intensity efficiently.
As described above, when a light intensity of each of laser beams is controlled to form an electrostatic latent image by the use of the VCSEL, as the number of the laser beams increases, the frequency of control processes also increases. Therefore, it is not possible to take the advantages of high resolution and high speed sufficiently. In addition to the above disadvantages, there are negative effects of the increase in cost of an apparatus and maintenance. Thus, there is a need of an image forming apparatus and an image forming method that can make it possible to form an electrostatic latent image at high resolution and high speed by optimizing the control of light intensity with characteristics of a VCSEL effectively.
Furthermore, conditions for forming an image vary in accordance with an environmental temperature around an image forming apparatus or with the passage of time. Therefore, it is necessary to adjust a light intensity of each of laser beams emitted from the VCSEL in accordance with the changed conditions before forming an image. However, as described above, because the number of laser beams to be adjusted is large in case of the VCSEL, an error in adjustment of the light intensity among the laser beams is prone to occur. As a result, an uneven image density (banding) periodically appears in the printed image. Therefore, there is a need of an image forming apparatus and an image forming method that can make it possible to reduce a light-intensity deviation among laser beams when a light intensity of a VCSEL is adjusted.
The present invention has been focused on a point that, when using a VCSEL in an image forming apparatus, the cause of the problem is that the light-intensity control technology used in the conventional semiconductor laser is simply extended and applied to the VCSEL.